Shortly after the isolation and characterization of the first phytoalexins, it was shown that pycelial extracts of Monilia fructicola, a fruit tree pathogen, contained a peptide, referred to as monilicolin A, capable of inducing the accumulation of phaseolin (phytoalexin) in the endocarp of the beam phaseolus vulgaris [Cruickshank and Perrin, Life Sciences, 7, 449-458 (1968)]. Since this study, many eliciting preparations have been isolated, but they have essentially been partially purified fractions. In effect, the purification of elicitor molecules from crude fungal extracts is complicated by response curves that are multiphasic or variable according to the dose applied, the partial or total loss of activity, the response resulting from the interaction of different compounds present in the mixture, or quite simply the chemical nature of the elicitors.
In the Phytophthora capsici--Capsicum system, Capsicum cotyledons, kept alive on the fungal culture filtrate, exhibit a lower sensitivity with respect to this pathogen [Molot et al. Annales de Phytopathologie, 12, 95-107 (1980)]. An identical effect may be obtained using isolated fractions of culture filtrates or of the mycelium [Molot et al., Annales de Phytopathologie, 12, 379-387 (1980); Coulomb et al., Comptes Rendus de l'Academie des Sciences de Paris, series D, 290, 275-277(1980)].
Molot et al. have observed that, in capsicum, the reaction is not accompanied by any accumulation of phytoalexin [Agronomie, 4, 829-833 (1984); Phytopathologische Zeitschrift, 112, 268-276 (1985)]. Other defence mechanisms must hence be envisaged.